Corrosion resistant spacer

TECHNICAL FIELD

The present disclosure generally relates to corrosion resistance at a bi-metal junction.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Corrosion can occur at a junction between devices formed of dissimilar metals due to galvanic action. In general, at a junction between dissimilar metals, the metal with a more negative potential corrodes preferentially. By way of example, when a device formed of magnesium is in physical contact with a device formed of aluminum in the presence of a corroding environment, e.g., an electrolyte such as salt, the magnesium device corrodes near the junction. This is known as galvanic corrosion. Galvanic corrosion occurs because the corrosion electric potential of magnesium is about −1.6 volts while that of aluminum is about 0.8 volts. Hence, the magnesium device becomes an anode, the aluminum device becomes a cathode, and there is a current exchange including dissolution of the metal on the anode (magnesium) side.

A known way to protect such type of galvanic corrosion in metals is to provide electrical insulation between the two devices. But insulating materials like gaskets are not readily employable in certain applications, e.g., an automotive engine cradle subject to high temperatures and adverse loading conditions.

A liquid galvanic coating for protection of embedded metals has been proposed, wherein a fluid galvanic coating for protecting corrosion-susceptible materials is embedded within a substrate. The coating includes one or more metals selected from the group consisting of magnesium, zinc and alumina or more elements and/or one or more additives selected from the group consisting of conductive polymers, carbon fibers and graphite.

Another proposed manner of protecting embedded corrosion-susceptible materials requires coating of an overall structure with a conductive paint and applying current by the use of an external power supply. Such systems are costly to install, require a continuous power supply and must be periodically monitored and maintained throughout the life of the structure. Sacrificial cathodic protection methods typically require the application of metallic zinc.

Another proposed method includes applying a coating that acts as an electrolytic barrier and a cathodic corrosion prevention system, applicable to ferrous and non-ferrous metal substrates. The method provides cathodic protection from corrosion by coating with polymers and sacrificial anodic metal particles. This coating system is formed by a process that includes premixing of an inherently conductive polymer with anodic metal particles to form an inherently conductive polymer/metal particle complex.

Another proposed method to protect metals from corrosion uses a type of coating called barrier coating. Barrier coatings function to separate metal from the surrounding environment. Some examples of barrier coating include paint and nickel and chromium plating.

Another type of coating used to protect metal is called sacrificial coating. The metal is coated with a material that reacts with the environment and is consumed in preference to the substrate it protects. These coatings may be further subdivided into chemically reactive, e.g., chromate coatings and electrochemically active or galvanically active, e.g., aluminum, cadmium, magnesium and zinc. The galvanically active coatings must be conductive and are commonly called cathodic protection.

There can be difficulty in creating a coating that protects like a cathodic system but is applied with the ease of a typical barrier coating system. Furthermore, there are environmental considerations related to plating operations and surface preparation for certain top coating processes.

Metallic spacers have been used in automobiles. In one example, an aluminum spacer has been placed between magnesium and steel, creating a junction consisting of magnesium-aluminum-steel. This junction generates electrochemical corrosion potentials of Mg: −1.6V/Al: −0.8V/Fe: −0.2V. The electrochemical corrosion potentials suggest that if the corrosion potential of a spacer has an intermediate value of the other components of the junction, then the galvanic corrosion is reduced. It has been found that a single alloy spacer is not substantially effective in preventing galvanic corrosion, particularly in automotive components formed of magnesium and exposed to high temperature operating environments.

SUMMARY

The present disclosure sets forth an interface device insertable at a junction between a first device comprising a first metal and a second device comprising a second metal that is dissimilar to the first metal. The interface device comprises at least one layer comprising an alloy of the first metal and the second metal and having a functionally gradient composition operative to reduce the galvanic effect between the first and second devices.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

<figref idrefs="DRAWINGS">FIG. 1</figref> is a schematic illustration, in accordance with the present disclosure;

<figref idrefs="DRAWINGS">FIG. 2</figref> is a schematic illustration, in accordance with the present disclosure; and

<figref idrefs="DRAWINGS">FIGS. 3-5</figref> are datagraphs, in accordance with the present disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same, <figref idrefs="DRAWINGS">FIG. 1</figref> schematically illustrates a junction <b>10</b> of devices of dissimilar metals that has been constructed in accordance with an embodiment of the disclosure. The junction <b>10</b> includes a first device <b>20</b> and a second device <b>40</b>, with an interface device <b>30</b> inserted therebetween. The junction <b>10</b> as depicted is secured via a fastener <b>50</b>, depicted as a pass-through bolt which compressively connects the first device <b>20</b>, the interface device <b>30</b>, and the second device <b>40</b>. A first subjunction <b>25</b> is formed between the first device <b>20</b> and the interface device <b>30</b>, and a second subjunction <b>35</b> is formed between the interface device <b>30</b> and the second device <b>40</b>.

The first device <b>20</b> is formed from a first metal, in this embodiment comprising aluminum (also referred to herein by its element symbol Al). Aluminum has a corrosion potential of approximately −0.8V. The second device <b>40</b> is formed from a second metal dissimilar to the first metal, in this embodiment comprising magnesium (also referred to herein by its element symbol Mg). Magnesium has a corrosion potential of approximately −1.6V. The interface device <b>30</b> is formed from bimetallic alloys of the first metal and the second metal, i.e., Al and Mg.

The interface device <b>30</b> comprises a spacer composed of a plurality of layers <b>32</b>, with each layer <b>32</b> formed from a bimetallic alloy of the first and second metals. The alloys of the layers <b>32</b> of the interface device <b>30</b> are formed to effect a functionally gradient composition with regard to galvanic current densities generated at the first subjunction <b>25</b> and the second subjunction <b>35</b>. The layers <b>32</b> are preferably composed in a gradation of the alloys to achieve a galvanic current distribution which minimizes an effective galvanic current peak at the one of the first and second subjunctions <b>25</b> and <b>35</b> between the interface device <b>30</b> and the one of the first and second devices <b>20</b> and <b>40</b> formed from the one of the first and second metals having the lower corrosion potential. The gradation of alloy composition of the layers <b>32</b> can be achieved by forming the layers with varying alloy compositions such that the layer <b>32</b> adjoining the first device <b>20</b> comprises an alloy having a relative maximum percentage of the first metal and the final layer adjoining the second device <b>40</b> comprises an alloy having a relative minimum percentage of the first metal. Preferably, the each of the aforementioned alloys of the interface device <b>30</b> is composed such that the one of the first and second metals having the lower corrosion potential predominates. Thus, in the first embodiment described hereinbelow with reference to <figref idrefs="DRAWINGS">FIG. 2</figref>, the layers <b>32</b> are formed from Mg—Al alloys in gradations which minimize galvanic peak current at the second subjunction <b>35</b> between the interface device <b>30</b> and the second device <b>40</b> formed from magnesium, which has a lower corrosion potential than aluminum.

<figref idrefs="DRAWINGS">FIG. 2</figref> schematically depicts an embodiment of the interface device <b>30</b> constructed of three layers <b>32</b>, depicted herein as a first layer <b>32</b>A, a second, intermediate layer <b>32</b>B, and a third layer <b>32</b>C. The first layer <b>32</b>A adjoins the first device <b>20</b> at the first subjunction <b>25</b>. The third layer <b>32</b>C adjoins the second device <b>40</b> at the second subjunction <b>35</b>. The layers <b>32</b>A, <b>32</b>B, and <b>32</b>C are formed from bimetallic alloys of the first and second metals, with a gradation in the composition thereof. As depicted, the first layer <b>32</b>A is formed from an alloy referred to as AM100, comprising 10%-Al/90%-Mg. The second layer <b>32</b>B is formed from an alloy referred to as AM80, comprising 8%-Al/92%-Mg. The third layer <b>32</b>C is formed from an alloy referred to as AM60, comprising 6%-Al/94%-Mg. In this embodiment, each of the layers <b>32</b>A, <b>32</b>B, and <b>32</b>C is approximately 1.0 mm thick, with the interface device <b>30</b> having a total thickness of approximately 3.0 mm. The interface device <b>30</b> can be produced by joining alloy sheets formed from materials of the multiple layers <b>32</b>A, <b>32</b>B, and <b>32</b>C using a hot-rolled process analogous to metal cladding or by another process. Alternatively, the interface device <b>30</b> can be composed of various quantities of layers <b>32</b>, e.g., a single layer, two layers, four layers, of varying alloys of the first and second metals to achieve a gradation of composition of the layers <b>32</b> to achieve a galvanic current distribution which minimizes an effective galvanic current peak at the one of the first and second subjunctions <b>25</b> and <b>35</b> between the interface device <b>30</b> and the one of the metals having lower corrosion potential, taking into account other factors, e.g., physical space requirements, application environment, and service life.

<figref idrefs="DRAWINGS">FIG. 3</figref> graphically depicts corrosion potentials (Volts) and galvanic current densities (A/m²) for a plurality of exemplary metal and bimetal alloy compositions, including intersections A, B, and C occurring therebetween. The compositions include aluminum (depicted as ‘Al’), magnesium (depicted as ‘Mg’), an alloy comprising 10%-Al/90%-Mg (depicted as ‘AM100’), and, an alloy comprising 30%-Al/70%-Mg (depicted as ‘AM300’). The data indicate that intersection A formed between Mg and Al yields a maximum galvanic current density, as compared to intersection B formed between Al and AM100, and intersection C formed between Al and AM300. The results indicate that galvanic current density decreases at an intersection between a device formed of Al and an interface device <b>30</b> composed of an alloy comprising Al/Mg.

<figref idrefs="DRAWINGS">FIG. 4</figref> graphically depicts experimental results for alternate embodiments. The use of similar numerals indicates use of similar devices as have been previously described. In the experimental system, several junctions <b>10</b> were constructed, comprising the first device <b>20</b>, the interface device <b>30</b>, and the second device <b>40</b>. In this embodiment, the first device <b>20</b> is formed from steel, having a corrosion potential of approximately −0.2V, and the second device <b>40</b> is formed from magnesium having a corrosion potential of approximately −0.8V. A plurality of interface devices <b>30</b> were constructed, formed from a single Mg—Al layer <b>32</b>. A junction consisting of only the first device <b>20</b> and the second device <b>40</b> (not shown) was also constructed. Each of the constructed junctions was tested for galvanic corrosion using a galvanic corrosion test. Test results of the galvanic corrosion test comprise a corrosion depth D in the second device <b>40</b> proximal to the junction <b>10</b>, compared to a pre-test depth (nominally 0.0 mm). Each of the interface devices <b>30</b> comprised a single layer <b>32</b> composed of alloys of aluminum (‘Al’) and magnesium (‘Mg’), compositions including 2%-Al/98%-Mg (‘AM20’), 3%-Al/97%-Mg (‘AM30’), 4%-Al/96%-Mg (‘AM40’), 5%-Al/95%-Mg (‘AM50’), 6%-Al/94%-Mg (‘AM60’), 7%-Al/93%-Mg (‘AM70’), 8%-Al/92%-Mg (‘AM80’), 9%-Al/91%-Mg (‘AM90’), and 10%-Al/90%-Mg (‘AM100’). The test results indicate that the interface device <b>30</b> composed of 7%-Al/93%-Mg (‘AM70’) yielded a minimum corrosion depth D for the compositions tested.

<figref idrefs="DRAWINGS">FIG. 5</figref> graphically depicts magnitude of galvanic current density occurring at the junction <b>10</b> between the first device <b>20</b>, composed of Al, and the second device <b>40</b>, composed of Mg plotted against a linear distance, in meters (m), from the junction <b>10</b>. The graphically depicted data was generated using known mathematical modeling and simulation techniques. Data for the galvanic current density is shown for the junction <b>10</b> having the interface device <b>30</b> comprising a single layer composed of an alloy of Al—Mg (‘One-layered Spacer’), and junction <b>10</b> having the interface device <b>30</b> comprising two layers composed of different Al—Mg alloys (‘Two-layered Spacer’), and a junction <b>10</b> having no interface device (‘Without Spacer’). The results indicate a decrease in galvanic current density at the second device <b>40</b> when using the interface device <b>30</b> comprising the two layers composed of different Al—Mg alloys. The corrosion potential and galvanic current density of the interface device <b>30</b> to achieve a functionally gradient composition with regard to galvanic current densities can be determined by modeling and numerical simulation, and is applicable to various combinations of first and second devices <b>20</b> and <b>40</b> composed of dissimilar metals, e.g., magnesium, aluminum, and steel.

It is understood that all references to specific metals and alloys are meant to be exemplary as descriptive embodiments of the disclosure, and not limiting. The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.